A comminuting machine of this kind, which is however only intended for scrap metal, is described in DE-OS 22 25 916. In this known comminuting machine the construction in the region of the grate , or of the grate arranged in the outlet region, indirectly or directly affects the result of the comminution. Such machines are however less suitable for use in the field of waste and useful material processing, since the specific requirements as to the quality of the end product, for example with regard to different particle sizes or degree of fineness and/or density, can only be met by the deliberate employment of different outlet grates with different grate openings, which must be exchanged to meet the desired end result of the comminution. The down-time of the machine associated with each change of grate and the resulting loss of production of course have an uneconomic effect, since the total down-time can become considerable, depending on the frequency with which a change of grate is required for the desired result of the comminution or for a particular comminution treatment. Further negative factors, in addition to the down-time already mentioned, are the long slowing-down time of the rotor, which is generally heavy and must come to a standstill before each change of grate, and the relatively high labour costs associated with the grate charging or with the modification of the machine.